The present invention relates to interposers and a method of mounting an integrated circuit chip (IC) or similar device using such an interposer connected across a gap between thin film circuits, such as two conductive film sections of a conductive film antenna, or similar circuit made from conductive film or other conductive flex circuit. More specifically, the invention is directed toward a method of mass-producing devices, such as radio frequency identification tags (RFIDs), by mounting integrated circuit chips (ICs) on interposers that are then physically and electrically connected to the circuit, such as a conductive film antenna, using a pressure sensitive conductive adhesive.
Radio frequency (RF) devices such as RFIDs can be used for inventory management, highway toll express passes, and many other purposes that are suited for interrogation of the device. Such devices can be made by incorporating an appropriate IC across the gap of two conductive surfaces of a thin conductive film antenna. Such an antenna may be formed by introducing a gap between a conductive film surface, thus creating two conductive portions or halves. The IC contains encoded data for identification purposes. The IC and the antenna act together as a transponder, which receives an RF signal and modifies it according to the data encoded on the IC.
The invention is an interposer that includes a thin substrate having two printed conductive ink pads. These pads provide a larger effective electrical contact area than ICs precisely aligned for direct placement without an interposer. The larger area reduces the accuracy required for placement of ICs during manufacture while still providing effective electrical connection. The substrate is coated with a pressure sensitive conductive adhesive that allows an IC to be electrically and mechanically mounted across the conductive ink pads. Pressure sensitive adhesives are preferred, but this production method is not limited only to pressure sensitive applications. The IC can be mounted across the gap of the interposer, ideally with the same adhesive as used to mount the interposer-IC subassembly to the circuit.
Methods are known in the art of connecting ICs to thin metallic foils, e.g., leadframes. In these methods, the IC is affixed on top of the leadframe, then the leadframe is connected to electrical connections on the IC by wire bonding techniques. Because of the fragile nature of the bonding, the connections are encapsulated for support. The present invention uses an interposer having conductive adhesive to both mechanically join and electrically connect the IC to a thin metallic film antenna, thus maintaining flexibility and providing a more robust connection. However, the scope of the present invention includes use in any thin film circuit application requiring placement of an IC or other flush mount components.
The prior art methods have difficulty in aligning ICs with base substrates or vice versa. IC placement was accomplished by first placing the individual base substrate in alignment with an IC, such that the bonding leads of the substrate are aligned to the connectors on the IC. Alternately, if the substrate is the base piece, the IC had to be precisely aligned for a physical and electrical connection. The relatively small bonding leads or pads on the substrate and IC provide a small effective connection area and greatly restrain the tolerances for placement of an IC across the gap of a thin metallic film circuit substrate. IC placement and mounting are serious limitations for high speed manufacturing.
A prior method of making a RF tag includes using a chip placement machine that mounts ICs between two separate conductive portions of a thin film antenna. This method requires the precise alignment of the IC between a gap of approximately 0.017 inch between the antenna portions. Such tolerances greatly reduce line speeds. This method is acceptable for high-priced items that can be produced at slow line speed. The chip placement machine cannot be readily used for high speed application of ICs on low cost devices. Also, a chip placement machine is relatively costly to purchase and operate. The interposer with a pre-positioned IC overcomes this required precision so that a convertor, such as a label printer, can affix ICs with a high line speed as required for uses such as packaging or other disposable products incorporating RFID tags or similar devices. A converter can use existing technology to apply the IC mounted on an interposer without the need to purchase a chip placement machine.
Previous methods also required bonding such as thermosonic; lasersonic; soldering; or wire bonding. Such methods typically require more process steps and often have defects due to the application of heat in soldering or wire bonding. Further, wave soldering or reflow ovens often expose components to high temperatures that may cause damage to the components. These difficult methods are replaced by applying an interposer of the present invention having a pre-positioned IC to the base substrate (i.e., antenna halves) with conductive adhesive. No heat needs to be applied in placing and connecting the interposer-chip subassembly to the substrate circuit. Heat and high humidity can be detrimental to chips. The pressure sensitive anisotropically conductive adhesive used in the present invention overcomes these bonding limitations and potential defect causing steps.
Current technology relies on the use of a conductive tape applied as a free film to attach the interposer to the RFID antenna or similar circuit. Significant flexibility, improved efficiency and lower cost are obtained by using a printable conductive adhesive. To form an aperture in a tape, a relatively difficult step of punching a hole is required. The adhesive of the present invention is printed over the conductor area and serves to attach the interposer to the antenna halves, both physically and electrically. It may be printed or applied in any pattern including having an aperture or recess.
Other techniques for bonding ICs to conductive circuit patterns of a substrate are known in the art. One such method uses an anisotropically conductive adhesive between raised areas (pads) for bonding on the substrate and matching bonding raised areas of the IC. The adhesive was typically an insulative polymer containing conductive particles that simultaneously contact a raised connection of the IC and a matching raised area of the substrate to provide interconnection. The conductive particles do not conduct in the lateral or horizontal direction since they transmit current only in the vertical direction between the substrate and device bonding raised areas. Such conduction is “anisotropic.” The polymer is cured after mounting the IC on the substrate that thereafter provides a permanent structural bond in addition to a conductive connection. The replacement of solder bond connections by anisotropic conductive adhesives can often reduce assembly costs, can accommodate relatively high concentrations of conductive connections, and can make devices more amenable to repair and reconstruction. Similar techniques are needed to bond ICs to thin film circuit substrates, such as antenna halves, without significant curing times, without excessive heat, and without matching bonding raised areas in precise alignment. The present method uses pressure sensitive anisotropically conductive adhesive between the IC mounted on an interposer and the antenna halves (base substrate) to solve these concerns. Pressure sensitive adhesives are selected for ease of application, but other adhesives are available though not preferred.
Prior adhesives with conductive particles that simultaneously contact a raised connection of the IC and a matching raised area of the substrate to provide interconnection were not used with interposers and required precise alignment of the components and were only usable in one orientation. Also, many disposable substrates, such as thin polymeric films, cannot tolerate the high temperatures associated with curing.
The ICs were previously encapsulated by the application of a chip face encapsulant material on the ICs that are attached to the antenna. Typically, an epoxy was used. Besides long curing times, the use of epoxy may cause undesirable friction when joining circuits to an interposer. The present interposer can secure the IC, rather than placing an epoxy encapsulant over the surface of the IC. The curing of an epoxy encapsulant often takes hours and is not conducive to high speed production. The present invention overcomes the long cure time of such encapsulants by using a pressure sensitive conductive adhesive and an interposer that can secure the IC. Also, filled epoxy is more rigid so devices requiring flexibility are not well suited for epoxy encapsulants over the surface of the IC. The interposer of the present invention maintains flexibility.
ICs have become drastically less expensive, and are often not the most expensive part of a device. An IC may be incorporated into a device, such as an RFID tag, a luggage tag or prescription label, in which the device substrate would be the most expensive component. The “expensive device substrate,” for lack of a better term, can be spoiled if the IC is not mounted correctly or if there is a short circuit in the mounted IC. Isolating the IC placement to the less expensive interposer subassembly allows IC defects to be detected before the final assembly to the more expensive device substrate, such as a thin film antenna. An IC on an interposer will make for less spoilage of the “expensive device substrate.” Spoilage is limited to the lowest cost component when a low-cost interposer with an IC is used.
This relates to another critical problem overcome by the interposer-chip combination. Certain applications require sequential production of expensive device substrates, such as numbered pharmaceutical labels or packages. If a bad IC is inserted in such a sequence-critical application, the web would have to be re-spliced, effectively shutting down production until completed. By isolating the individual interposer-chip combination as a subassembly, the interposer can be applied by a standard head that can repeat without shutting down when a bad IC is detected so that there are no missing labels or packages in the sequence.
It has therefore been found beneficial to produce an interposer that is inexpensive to mass produce and easy to insert at high line speed, preferably in multiple orientations with respect to the antenna or similar circuit. The present invention incorporates some of the advantages of the existing technology, while alleviating several of the problems associated therewith. Interposers are well suited for high volume applications because they offer low-cost solutions and easy attachment. Additionally, the improved method of manufacturing a thin RF transponder with such an interposer is necessary for such mass production.